DSLR Basics - what exposure, aperture, ISO etc

Hi guys,
I spent about a half hour tonight (before some hazy cloud came in) taking some test shots with the 350D. This was my first proper night time attempt. I took varying exposures in steps of 15 seconds starting from 30 seconds on the Cygnus cross. I was using the 28 to 80mm lens at F3.5, ISO 800. Granted I'm on the edge of town, with oncoming car lights illuminating the upper skies in my garden, but I was please that very little in terms of LP I got when pointed at the zenith.

The Results: I found that there was even slight trailing at 30 seconds. I also noticed that with each increased exposure, the trails inevitably got longer, but the brighter stars appeared slightly foggy. This fog however did not also increase with the exposures i.e. the 75 second shot had more bright star fog that the 105 second shot.

For manual exposures, what is recommended with my set up? And, without actually showing the photos, what can I do better from my first attempt to try on my second by way of how I described what I did?

All tips and suggestions apreciated.

Seanie

The rule of thumb is that with a 50mm lens on a DSLR the longest exposure that will not show trailing is 10 seconds. This can be pushed to 13 or even 15 seconds where the camera is aimed towards the pole where angular rotation is less.

There is an inverse relationship between focal length and allowable
shutter speed so that with a 24mm lens 20 secs will give good results, but a 135mm lens will be restricted to 4 secs. At 300mm the Earth's rotation is apparent at anything much over 1 second exposure!

Also, here are the slides of a talk I gave on the subject last year...

www.pevans.me.uk/html/don_t_be_afraid_of__m_.html

Hi Seanie,
The fogging you saw is probabily just local high haze conditions,although look out for dew on the camera lens.
As you probabily know, a lower F stop will yield you brighter stars as will a higher ISO rating, BTW, what camera are you using?, most people step down the F stop one notch so that the outer extreme edge of the camera lens is not used, these areas are usually the worst optically and can give distortion, experimenting here will tell you if thats worth it for your lens.

Using longer focal length lens is inevitably going to give you star trails. I personally dont mind star trails, I wouldnt get hung up about them, if anything, I'd go for longer trails, one thing I've never seen on the boards is a long exposure of the polar area showing rotation around polaris, this types of pictures are very striking.

If you want long exposures for constellations, you have to get some means of tracking.

Hi Seanie, as Paul & Dave have said, there are a number of factors that will have an effect on your final image. If you don't mind the star trails, you can go about not tracking the stars for now. There are

1.Camera settings
a) ISO - this number ranging from 100-1600 measuresd the cameras ability to respond to light. At 100, the camera is least sensitive to light, so astronomy wise, most of us with a dslr would use 400 or 800 when it's dark, Be minful though that higher ISO gives much more noise electronically which is also affected by temperature, the warmer, the more noise
b) F number - the higher the number, the more light is allowed into the sensor, but for sharp stars, the lens can be 'stopped' or put down a nothch or two, generally I find for astro shots the anything below f5.6 will not allow enough light in to record a bright image.
c) Exposure time - speaks for itself, the more time the brighter the image as a rule of thumb, but more trailing if untracked
d) Focal length of lens - this is the distance from the lens to the point where the image is in focus, the longer the length, the 'smaller' the image scale, or more magnified the image will be. So a 18mm lens will shoot widefield, while a 200mm lens will shoot a small area of the sky

There is also two more things - the quality of the lens and regarding zoom lenses, I know I can only speak for canon, but their 'L' lenses are top quality, while all others are of slightly lesser quality. Also zoom lenses are *generally* regarded as being not as good as 'prime' or fixed focal length lenses as their f numbers are lower at maximum aperature, thus leaving less light in.

2. What you want to shoot?
Well that's up to you, things to consider are light pollution, tracking (the longer the focal length of your lens, the more critical polar alignment), and of course your target.

Don't worry about being at the experimental stage, I was myself, and everything that I have written here, I have learned myself , and so will you with a little practice !

Keith..

one thing I've never seen on the boards is a long exposure of the polar area showing rotation around polaris,

Ask and thow shall receive:




Not particularly striking, twas just a test shot.
I photoshopped the bejasus out of the colour as the LP was God Awful.
I have better ones of us up on the hill in Boher but they are in the back log of
'to be processed' file on my PC.

Seanie,
The 'fogging' around the stars (I'm taking for granted you know what high haze or actual fog looks like ona long exposure) is probably due to the
focus not being absolutely bang on.
There is an artical in this months S&T which explains why this happens nicely and also it's be handy for you if you are just starting out with a dslr.
(nice choice BTW! )

I always find that you get noticable trailing after around 15 seconds.
I think trailing is totally fine on wide angled 'starscapes' as I call them.
If you want the really 'deep' look then slap the camera on a scope.
A lens hood will also help with dew / fogging on the lens.

Happy snappin!